1. Field of Invention
The present invention relates generally to the field of inductive sensors and more particularly to hybrid inductive speed and proximity sensors that measure speeds approaching zero.
2. Background Information
Rotary and linear inductive sensors, such as the variable reluctance speed sensor (VRSS) as shown in FIG. 1, or variable inductance proximity sensors as shown in FIG. 2 or induction sensors using permanent magnets on the target as shown in FIG. 3, have many years of reliable service. However, conventional, manned flight approved sensors can not accurately measure speeds approaching zero and have difficulty measuring the speed of targets without surface irregularities and also require close proximity between the sensor and the target. Variable reluctance speed sensors and induction sensors, either rotary or linear, require movement of a scrutinized target to generate a signal used to monitor speed. Accordingly, there is a target speed below which variable reluctance or induction speed sensors may not be useful.
Many variable inductance sensors may not have a permeable pole piece. This has consequences when using the typical variable inductance sensor with an induction-style permanent magnet on the target. Many variable inductance sensors that incorporate external AC excitation often require the sensor to be within approximately ½ of a coil diameter of the target that is being monitored to accurately measure speed or proximity. Assuming the permanent magnet is too far away to be influenced by the high frequency field coming from the sensor, the magnetic field from a permanent magnet on the rotating target may have no influence on a variable inductance sensor without a permeable pole piece. Thus, a magnet on the rotating target may be of no help in facilitating the use of the typical variable inductance sensor for zero-speed or proximity measurement across relatively large gaps or through significantly thick or dense conductive material. Sensors having a ferromagnetic or diamagnetic pole piece may be needed to sense the field from the permanent magnet.
What is needed is a zero speed sensor and or proximity sensor having proven manned flight safety and reliability.